This invention relates to a storage system used as an external storage system for computers, particularly to a magnetic disk unit the recording capacity, reliability, and access speed of which are variable, or to a personal computer used as a storage system that can select an external storage system having recording capacity, reliability, and access speed properties different from those of the current one.
Each of the conventional magnetic disks has recorded/reproduced every type of information with one method (quality and reliability); the recording/reproducing method is not changed according to the type (difference) of object information. Any one of those magnetic disk units cannot change its characteristics so that it can use the most suitable recording system (method) selected according to the type of object information.
For example, as shown clearly in J. M. Harker, et al, A Quarter Century of Disk File Innovation, IBM J.Res.Develop, Vol.25 (1981), p.667, etc., characteristics of a magnetic disk unit are fixed and its recording characteristics cannot be changed according to the type of object information.
Concretely, such a magnetic disk unit is used with the same reliability (low recording/reproducing error rate) to cope with (1) numerical data that requires high reliability and (2) image data that does not require as high a reliability as the numerical data, but requires large capacity. As a result, they cannot record mass information. Furthermore, they record both data that requires high speed property and data that does not require such high speed property with the same storage characteristics. This is why they are limited in their high speed recording/reproducing performance.